This invention relates to circuits for actuating apparatus, particularly tape recorders, beep-tone generators, lamps, relays or the like when the high on-hook voltage of the telephone line drops to or near low off-hook voltage, when the customer lifts the handset; in addition, line voltage drops from on-hook during negative swings of ringing voltage. In the prior art, actuation of such equipment was accomplished mainly by series relays energized by off-hook current. It was also accomplished by a relay in parallel with the line that necessarily drew high on-hook current and interfered with service. A vacuum tube was successfully employed for this service with a relay circuit employing two power sources. A transistor circuit to light a supervisory lamp approached this result with an additive voltage power source drawing relatively large amounts of on-hook current and was not set out accurately.
Circuits that actuate recorders in off-hook produce recordings that are more acceptable as legal evidence than those made with voice actuated (VOX) devices since there is no cut off of beginning phrases by circuit delay. This invention draws little or no current in on-hook and does not interfere with telephone service and draws little current in off-hook.
It is an object of this invention to actuate apparatus such as a magnetic tape recorder, principally the motor, or the motor and amplifier with a transistor in series with the apparatus and a power source, that may eliminate a relay, simplifies switching and is especially useful in telephone answering sets and dictating machines, including drum, disc and belt recorders. See U.S. Pat. Nos. 3,794,764 and 3,794,767.
It is an object of this invention to actuate such transistor either directly by the line voltage change or by another semiconductor switch such as an FET or another bipolar transistor, in arrangements that draw little or no current from the telephone line.
It is an object of this invention to increase the control element voltage breakdown (gate-source or base-emitter) by the use of a diode in the control element circuit, or the power circuit.